A sunspot is formed as a result of the internal magnetic field bursting through the visible surface and out into the corona. Sunspots appear dark because the magnetic fields get in the way of energy and heat being transported from inside the Sun to its surface. Because these areas are heated less, they are a few thousands of degrees Kelvin cooler (1,000-2,000K normally), making them appear dark in comparison to the very bright rest of the surface.
Sunspots are generally where flares and CMEs originate from. They are therefore used as marker of solar activity. Lots of sunspots generally means lots of solar flares and CMEs. Few or no sunspots mean little or no solar activity. Read more...